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## open delta

Is open delta trasformer considered one phase or two
How two phasa looks like

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An open delta is a three phase system.

3. I know it does not look like it could be 3-Phase with only two single phase transformers, but it is.
See this website:
http://www.3phasepower.org/3phasetransformers.htm

4. Cheap, economical, and efficient 3 phase service. Less power, 57% I think.

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Originally Posted by jumper
Less power, 57% I think.
Old wives tale, kind of.

If you have a bank of three transformers and then remove one of them, your are correct the result will be 57.7% of the designed amount you started with.

However, a bank designed using only 2 transformers can deliver 100% of its design.

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## Open-Delta Transformer Connections

As a transformer bank the use of open-delta is relatively inefficient where most of the loads are three-phase since the bank has only 86.6% of the rating of the two units making up the three-phase bank. It also has only 57.7% of the three-phase rating of the closed delta bank of three units. When the open-delta bank is used because the three-phase loads are small, you find transformers of different sizes being used. A larger transformer for the single-phase load and a smaller transformer for the lesser, three-phase loads.

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While we are on this subject, Where (cos 30º) =.866, Where does the 30º shift come from? Why is it there?

8. Originally Posted by jim dungar
Old wives tale, kind of.

If you have a bank of three transformers and then remove one of them, your are correct the result will be 57.7% of the designed amount you started with.

However, a bank designed using only 2 transformers can deliver 100% of its design.
Originally Posted by grant
As a transformer bank the use of open-delta is relatively inefficient where most of the loads are three-phase since the bank has only 86.6% of the rating of the two units making up the three-phase bank. It also has only 57.7% of the three-phase rating of the closed delta bank of three units. When the open-delta bank is used because the three-phase loads are small, you find transformers of different sizes being used. A larger transformer for the single-phase load and a smaller transformer for the lesser, three-phase loads.
This is true with three transformers of the same size and then you remove one. There are cases where there is larger transformer for the two 120 volt to ground phases because there is more load on those phases - then 57% changes even more, or there are even three transformer banks that still have a larger unit for 120 volt phases but two smaller units for the others - again because the 120 volt load is expected to be larger than the loads connected to the 208 volt phase.

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Originally Posted by kwired
This is true with three transformers of the same size and then you remove one. There are cases where there is larger transformer for the two 120 volt to ground phases because there is more load on those phases - then 57% changes even more, or there are even three transformer banks that still have a larger unit for 120 volt phases but two smaller units for the others - again because the 120 volt load is expected to be larger than the loads connected to the 208 volt phase.
Could you explain how the 57.7% changes based on the size of XF's? If I need more than a 25 kVa for the power pot, it needs to be closed IMO.

10. Originally Posted by Hv&Lv
While we are on this subject, Where (cos 30º) =.866, Where does the 30º shift come from? Why is it there?
Are you talking about the delta/wye 30º phase shift?

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